Required of electrolyte membranes for fuel cells are high ion conductivity, high strength, high toughness, high heat resistance, high durability, low swell, low fuel permeability, low cost, low environmental load and the like. Especially in direct methanol fuel cells, a high ion conductivity, high strength, high toughness, low swell, and low fuel permeability are required. With respect to such electrolyte membranes for fuel cells, studies have been made on perfluorosulfonic acid polymers, sulfonated forms of aromatic hydrocarbon polymers as typified by sulfonated polyimides, sulfonated fluoro-polymers to which styrene is radiation grafted, and inorganic/organic hybrid materials.
The perfluorosulfonic acid polymers offer a high ion conductivity at a relatively low ion-exchange capacity, but suffer from a high fuel permeability due to an extremely high degree of swelling in methanol.
In the case of sulfonated polyimides, the ion-exchange capacity must be increased to acquire a high ion conductivity. The increased ion-exchange capacity, however, allows for swelling in water, with the drawback of an extreme drop of strength.
In the case of sulfonated fluoro-polymers having styrene radiation-grafted thereto, an extreme drop of strength does not occur even when the ion-exchange capacity is increased to acquire a high ion conductivity. However, the increased ion-exchange capacity allows for swelling in water, raising a problem of increased fuel permeability.
In the case of inorganic/organic hybrid materials, increasing a proportion of the inorganic component may be effective in restraining swell, but give rise to a problem of impairing toughness and becoming fragile. Those materials in which polyurethane is used as the organic component to increase toughness exhibit a high toughness in a dry state due to the hydrogen bond between urethane links, but suffer from an abrupt drop of strength upon water absorption because the hydrogen bond between urethane links is prohibited by water.                Patent Document 1: JP-A H08-501653        Patent Document 2: JP-A 2000-510511        Patent Document 3: JP-A H08-503574        Patent Document 4: JP-A 2003-100316        Patent Document 5: JP-A 2004-273386        Non-Patent Document 1: Industrial Material, April 2003, Nikkan Kogyo Shinbun        